There is a need for a torque sensor or indicator in the drive line to the rotary table of a drilling rig, to provide a warning when the torque needed to rotate the drilling string is rising--such a rise is frequently an indication that the drilling bit is seizing up.
A commonly used system for this purpose involves the use of a deflectable rubber wheel which is deflected by the return chain of the drive line. The wheel is moved against the resistance of a hydraulic indicator. One problem with this system is that the chain tends to cut into the rubber wheel until it reaches the wheel's steel hub, at which point damage occurs either to the chain or the hub. Still another problem is that the wheel and chain have to be aligned--a condition which is difficult to maintain. But the most significant problem is that the chain tends to flap, which causes the hydraulic indicator to show rapidly varying high and low values. Thus there is a time lag before the observer can identify a definite trend of increase in the indicated torque over the normal vibration of the indicator. So there are problems of endurance, maintenance and sensitivity associated with this type of sensor.
A second type of torque sensor in use on drilling rigs involves the use of strain gauges which actuate electronic signal means. This type of sensor has found only limited application, apparently because the electronic equipment tends to fail or becomes damaged in the rugged environment of a drilling rig.
So there is still a need for a torque sensor which:
is capable of monitoring high torque values; PA1 provides accurate indications of the torque; and PA1 is rugged, consistent and enduring, yet narrow and cylindrical in shape, so that it can be used in the crowded confines through which drilling rig drive lines extend. PA1 End to end shafts are provided, one of which is connected to the shaft of the motor and the other to the shaft of the load; PA1 Each of the shafts carry radial flanges at their adjacent ends. The motor shaft flange has cone-shaped recesses formed in its forward face, which recesses are formed with inclined plane surfaces. The load shaft flange has axial apertures which correspond with the motor shaft recesses; PA1 A ball seats in each recess and extends through the adjacent aperture; PA1 At its forward end, the ball abuts a thrust bearing which can be biased to compress a diaphragm and thereby pressure hydraulic fluid behind the diaphragm. This hydraulic fluid actuates an indicator, such as a Bourdon tube. PA1 as torque on the rotating motor shaft flange increases, the plane surfaces of the cone recess function to force the ball to move axially, thereby squeezing the diaphragm; PA1 the ball interconnects the two shafts to transmit torque, so that they rotate together, while still enabling the axial force, indicative of torque, to be expressed as a longitudinal mechanical movement which can be used to actuate a hydraulic indicator system associated with the stationary housing of the assembly.
There are other torque indicators of interest shown in the patent prior art. Of particular interest is U.S. Pat. No. 3,108,471, issued to Buchele et al. This patent discloses a sensor in which:
There are certain concepts inherent in this prior art device, which concepts are of interest in the present case, namely:
There are, however, certain problems associated with the Buchele et al design. More particularly, the spinning flange and ball assembly is sure to generate heat and this heat will expand the ball and the components connecting it with the diaphragm and cause the diaphragm to be compressed by the effect of thermal expansion. As a result, the indicated torque values will be misleading, being partly due to the thermal expansion. Also, the Buchele et al unit, due to its conceptual arrangement, would be short in length and wide in diameter, which would make it inappropriate for insertion in the crowded confines associated with the drive line of a drilling rig.
There are therefore further characteristics which would be desirable in a torque sensor, more particularly: it should be designed so that the torque readings are not affected by thermal expansion; and it should be relatively small in diameter.